This invention relates to micro-wave antennas in general and more particularly to an improved micro-wave antenna particularly useful in Doppler navigation systems.
A common problem in Doppler navigation antennas is what is known as over-water shift. Because of the different characteristics of returned energy from land and water, in the typical Doppler system, a shift occurs when flying over water which can lead to considerable error. One manner of overcoming this is what is known as a beam lobing technique in which each of the Doppler beams are alternated between two positions, a few degrees apart. Although such an approach has been found workable, it requires additional hardware and additional time. Another approach is that disclosed in U.S. Pat. No. 2,983,920 granted to R. H. Rearwin and assigned to the same assignee as the present invention. Disclosed therein is a planar array of micro-wave antennas which are slanted at 45.degree. to permit generating a beam shape which exhibits a high degree of independence from over-water shift. However, the implementation disclosed therein is not particularly practical.
Another problem encountered in antennas of this nature is an error caused by changes in frequency. This is a particular problem in printed antennas which are finding widespread use today because of their simplicity and low cost.
Thus, the need for an improved antenna of this nature which is not sensitive to over-water shift and is insensitive to frequency, becomes apparent.